Like a lot of other comments here, I agree that it's been moved to private chats, but I would add that what's missing is:

- everyone having a "Profile" that can be passively looked at at any time. Having an old classmate or a friend from your friend group (kind of solved in group chats) that's not a direct friend, but you still want to be part of that community - or have a sense of their life without having to the whole social interaction part.

- events

- the ability to "look up" someone before actually interacting with them

- the ability to "post on someone else's wall". This is something that many Facebook clones got wrong. I don't want to tag someone, I want to put something on their page that their friends can see that I put there.